The room-temperature structure of the B-site-ordered complex perovskite dicalcium magnesium tungstate, Ca2MgWO6, has been determined by simultaneous Rietveld refinement of neutron and X-ray powder diffraction patterns. Ca2MgWO6 is characterized by B-site ordering and an a(-)a(-)c(+)-type BO6 octahedral tilt mechanism.